The invention relates generally to the field of solid state photo-sensors and imagers, specifically imagers referred to as Active Pixel Sensors (APS).
Active Pixel Sensors (APS) are solid state imagers wherein each pixel contains a photo-sensing means, reset means, a charge transfer means, a charge to voltage conversion means, and all or part of an amplifier. APS devices have been operated in a manner where each line or row of the imager is selected and then read out using a column select signal (analogous to a word and bit line in memory devices respectively). In prior art devices the connection or contact to the various nodes within the pixels of a given row are done on a per pixel basis, even tough they are the same electrical node within a row (see FIG. 1). Since contact regions are placed in each pixel, and contact regions typically consume large amounts of pixel area due to the overlap of metal layers required, inclusion of these contact regions in each pixel reduces the fill factor for the pixel because it takes up area that could otherwise be used for the photodetector. Connection to each of these components to the appropriate timing signal is done by metal busses that traverse the entire row of pixels. These metal busses are optically opaque and can occlude regions of the photodetector in order to fit them into the pixel pitch. This also reduces the fill factor of the pixel. Decreasing the fill factor reduces the sensitivity and saturation signal of the sensor. This adversely affects the photographic speed and dynamic range of the sensor, performance measures that are critical to obtaining good image quality.
In order to build high resolution, small pixel APS devices, it is necessary to use sub-xcexcm CMOS processes in order to minimize the area of the pixel allocated tot he non-photodetector components in the pixel. In essence, it takes a more technologically advanced and more costly process to realize the same resolution and sensitivity APS device when compared to a standard charge coupled device (CCD) sensor. However, APS devices have the advantages of single supply operation, lower power consumption, x-y addressability, image widowing and the ability to effectively integrate signal processing electronics on-chip, when compared to CCD sensors.
From the above discussion it should be apparent that there remains a need within the art for an improved method of employing signal buss structures within APS devices.
The present invention is directed to overcoming one or more of the problems set forth above. Briefly summarized, according to one aspect of the present invention, an active pixel sensor comprising: a CMOS substrate having a plurality of pixels formed in a plurality of rows and columns; wherein each of the pixels further comprises a photodetector having active circuitry elements associated with the photodetector; at least two control busses associated with each row of pixels; and wherein one of the control busses for a given row functions one of the control busses for a different row.
One approach to providing an image sensor with the sensitivity of a CCD and the advantages of an APS device, is to improve the fill factor and sensitivity of an APS device. This invention does so by eliminating the need for a separate signal line contact area in each pixel, and using the timing signal and buss for one row as a timing signal and buss for the next row.
These and other aspects, objects, features, and advantages of the present invention will be more clearly understood and appreciated from a review of the following detailed description of the preferred embodiments and appended claims, and by reference to the accompanying drawings.
All of the features and advantages of prior art APS devices are maintained while requiring less pixel area for contact regions and metal busses. This provides the following advantages: Higher fill factor, sensitivity and saturation signal for the same pixel size, smaller pixel and device size for the same fill factor.